As the problem of environmental pollution increases, the competition among different technical fields to develop environmentally-friendly energy is increasing. In addition, the competition to develop alternative sources of energy due to a depletion of petroleum and natural resources, and the like has increased. Accordingly, the competition among vehicle manufacturers to develop next-generation vehicles is increasing. Among the next-generation vehicles, there are pure electric vehicles (EV) which use a battery as an energy source and an engine hybrid electric vehicle (HEV), a fuel cell hybrid electric vehicle, and the like, which use a battery as an energy buffer.
In the hybrid vehicle, a battery system is one of main parts that determines a quality of a vehicle. The battery system of the hybrid vehicle is an auxiliary energy source which assists an output of an engine and accumulates generated energy while the vehicle is driven. In particular, a control technology of the battery system is important which includes a power control, cooling, diagnosis, a state of charge (SOC) calculation, and the like.
Among those control technologies, the state of charge calculation is an important factor for driving strategy of a vehicle. In other words, in the hybrid vehicle, the state of charge is calculated to operate a battery to be charged when additional energy is generated and operate the vehicle to obtain a required output by discharging the battery when a substantially high output is required. Therefore, to reduce energy and maximize operation efficiency of the hybrid vehicle by accurately implementing the driving strategy of the hybrid vehicle, there is a need to more accurately calculate the state of charge.
When the state of charge calculation is inaccurate, the operation efficiency of the hybrid vehicle may be reduced and dangerous situations may occur. For example, when the actual state of charge is 80% but the calculated state of charge is 30%, a vehicle controller is configured to determine that the charging is required, and thus the battery may be overcharged or in the opposite case, the battery may be overdischarged. Ignition or explosion of the battery may occur due to the overcharging or the overdischarging, and thus dangerous situations may occur.
For the state of charge calculation, battery degradation, that is, a state of health is provided as a main input. The state of health is increasing degraded based on use environment or a use period of time than an early production of a battery, and thus an available capacity is reduced or resistance is increased. Generally, the state of health is degraded to about 20%. To prevent the reduction in energy and the risk by efficiently operating the hybrid vehicle, there is a need to more accurately estimate the state of health.
The existing method for estimating a state of health may vary, but may be divided into two methods. A first method is a method for directly applying a load having a predetermined frequency configuring hardware to the battery to measure impedance thereof and understand the state of health. A second method is a method for acquiring current and voltage pair data for a predetermined period to infer indirect impedance or a degraded capacity.
The first method for using hardware may have reduced efficiency due to errors, durability, costs, and the like of a method for configuring a circuit thereof and sensors. The second method may be difficult to implement the accurate inference or may have complex logic due to strong nonlinearity and disturbances from the method for acquiring the current and voltage pair data to the method for inferring the indirect impedance and the degraded state of charge.
Accordingly, a method for estimating a state of health by calculating charging capacity within a specific voltage section based on a current accumulation is developed, which may be applied at room temperature and during slow charging and may be vulnerable to accumulated errors of a current sensor during the current accumulation.